1. Field of the Invention
This invention relates to a spin filter preferably usable for quantum communication and quantum information processing.
2. Description of the Prior Art
Recently, much attention is paid to such research and development for quantum communication and quantum operation utilizing electron spin. In such a quantum technique, it is required to measure the spin directions of carriers to be utilized by means of unitary conversion when information is read. In order to measure the spin direction of carriers high sensitively, Time-Resolved Faraday Rotation Method has been developed and currently practically employed, but the operationality of the Faraday Rotation method is complicated because it requires a large-scale laser apparatus.
It is an object of the present invention to provide an element capable of easily measuring the spin directions of carriers to be utilized in such a technique as quantum communication and quantum operation.
In order to achieve the above object, this invention relates to a spin filter including a first magnetic semiconductor multi-quantum well structure, a second magnetic semiconductor multi-quantum well structure and a non-magnetic semiconductor quantum well structure which is located between the first magnetic semiconductor multi-quantum well structure and the second magnetic semiconductor multi-quantum well structure,
the first magnetic semiconductor multi-quantum well structure and the second magnetic semiconductor multi-quantum well structure being split in spin state,
whereby carriers in down-spin state are penetrated through the first magnetic semiconductor multi-quantum well structure and carriers in up-spin state are penetrated through the second magnetic semiconductor multi-quantum well structure.
This invention also relates to a spin filter comprising a magnetic semiconductor multi-quantum well structure and a non-magnetic semiconductor quantum well structure which is located adjacent to the magnetic semiconductor multi-quantum well structure,
the magnetic semiconductor multi-quantum well structure being split in spin state,
whereby carriers either in down-spin state or in up-spin state are penetrated through the magnetic semiconductor multi-quantum well structure.
The inventors have developed a spin filter composed of a magnetic semiconductor multi-quantum well structure and a non-magnetic semiconductor multi-quantum well structure as mentioned above in order to measure the spin directions of carriers.
For example, the magnetic semiconductor multi-quantum well structure can be made of II-V semiconductor compound, particularly as a multilayered structure composed of ZnSe layers and ZnMnSe layers which are stacked alternately. When the thicknesses of the ZnSe layer and the ZnMnSe layer are controlled appropriately, only carriers in down-spin state or up-spin state within a given energy range can be penetrated through the magnetic semiconductor multi-quantum well structure.
FIG. 1 is graphs showing the permeability of carriers in simulation when the thicknesses of the ZnSe layer and the ZnMnSe layer constituting the magnetic semiconductor multi-quantum well structure are varied. The simulation was performed under the condition that the ambient temperature was set to 4K and the strength of the magnetic field to be applied was set to 5T and the stacking periodic number was set to 10.
As is apparent from FIG. 1(a), when both of the thicknesses of the ZnSe layer and the ZnMSe layer were set to 5 nm, only the carriers in up-spin state within an energy range of about 20-30 meV can be penetrated and only the carriers in down-spin state within an energy range of about 30-40 meV can be penetrated.
Then, as is apparent from FIG. 1(b), when both of the thicknesses of the ZnSe layer and the ZnMnSe layer were set to 7.5 nm, only the carriers in up-spin state within an energy range in the vicinity of about 10 meV can be penetrated and only the carriers in down-spin state within an energy range of about 10-20 meV can be penetrated. In addition, as is apparent from FIG. 1(c), when both of the thicknesses of the ZnSe layer and the ZnMnSe layer were set to 10 nm, only the carriers in down-spin state within an energy range in the vicinity of about 10 meV can be penetrated.
Therefore, in the first spin filter according to the present invention, if ZnSe layers and ZnMnSe layers each having a slightly smaller thickness than 7.5 nm are stacked alternately to form the first magnetic semiconductor multi-quantum well structure, and ZnSe layers and ZnMnSe layers each having a thickness of 5 nm are stacked alternately to form the second magnetic semi-conductor multi-quantum well structure, as is apparent from FIGS. 1(a) and 1(b), the carriers in down-spin state within an energy range of about 20-30 meV can be penetrated through the first magnetic semiconductor multi-quantum well structure and the carriers in up-spin state within the same energy range can be penetrated through the second magnetic semiconductor multi-quantum well structure.
As a result, according to the first spin filter of the present invention, the carriers in down-spin state and up-spin state within the same energy range can be filtered simultaneously.
Moreover, if the first magnetic semiconductor multi-quantum well structure and the second magnetic semiconductor multi-quantum well structure are formed in similar configuration, the carriers in down-spin state and up-spin state within their respective different energy ranges can be filtered.
For example, if both of the first magnetic semiconductor multi-quantum well structure and the second magnetic semiconductor multi-quantum well structure are composed of the same multilayered structure where ZnSe layers and ZnMnSe layers each having a thickness of 5 nm are stacked alternately, as is apparent from FIG. 1(a), the carriers in up-spin state within an energy range of about 20-30 meV can be penetrated and the carriers in down-spin state within an energy range of about 30-40 meV can be penetrated.
If it is required that only the carriers either in down-spin state or up-spin state within a given energy range is penetrated, a single magnetic semiconductor multi-quantum well structure is provided, as a substitute for the first and the second magnetic semiconductor multi-quantum well structure.
For example, if a single magnetic semiconductor multi-quantum well structure where ZnSe layers and ZnMnSe layer each having a thickness of 5 nm are stacked alternately is provided, as is apparent from FIG. 1(a), only the carriers in up-spin state within an energy range of about 20-30 meV can be penetrated and only the carriers in down-spin state within an energy range of about 30-40 meV can be penetrated. The second spin filter of the present invention is conceived on the above-mentioned viewpoint.
As mentioned above, according to the first and the second spin filters of the present invention, Carriers can be selectively penetrated and thus, filtered. Therefore, the spin filters can be employed as a next-generation element usable for quantum communication and quantum operation.
In the present invention, the term xe2x80x9ccarrierxe2x80x9d means an electron or a hole in solid substance such as the magnetic semiconductor multi-quantum well structure, and generated by the irradiation of light or the application of electric field.